1. Field of the Invention
The present invention is related to integrated image sensors, and more particularly, the present invention teaches an image sensor that uses a wave-guide tube.
2. Description of the Prior Art
CMOS image sensors (CIS) are popular image sensors at present. CIS fabrication can be integrated into conventional semiconductor processes, and therefore CIS has advantages of low cost, tiny size, and high integration. CIS also has advantages of low operating voltage, less power consumption, high quantum efficiency, low read-out noise, and random access. Therefore, CIS is adopted broadly in electronic products, such as PC cameras and digital cameras.
A conventional CIS structure may be divided by function into a light sensing area and a peripheral electronic circuit area. The light sensing area has a plurality of photodiodes arranged in an array, and MOS transistors to sense light intensity, i.e. a reset transistor, a current source follower and a row selector. The peripheral electronic circuit area connects interconnects to external connections. A principle function of the CIS is to divide incident beams into combinations of light with different wavelengths. The light is received by a plurality of imaging devices on the semiconductor substrate and transformed into digital signals of different intensity. For instance, an incident beam is divided into a combination of red, green and blue light and received by corresponding photodiodes. Each photodiode transforms the light intensity into digital signals. In addition, the photodiodes handle the digital signals based on light currents generated in the light sensing area. For example, a light current generated during illumination is regarded as signal, and a dark current generated during darkness is regard as noise. Therefore, the photodiode compares the intensity of the signal and the noise, and accordingly transfers the result to the peripheral electronic circuit.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a traditional CIS structure, which includes a semiconductor substrate 10 and a plurality of photodiodes 11,12 and 13 formed therein. Each photodiode has an N-type region 16 and a P-type region 17 forming a device for sensing light intensity. Each photodiode is separated from other photodiodes by an array of shallow trench isolation (STI) structures 14. Thus, an array of pixels is formed. To achieve an intact CIS structure, multilevel interconnects are employed. For instance, the semiconductor substrate 10 is covered by a series of dielectric layers, such as an interlevel dielectric (ILD) layer 20 and intermetal dielectric (IMD) layers 22 and 24. Further, a wire pattern of interconnects (not shown) and metal lines 23 and 25 are formed in the IMD layers 22 and 24.
Incident light 32 and 34 from a light source 30 strikes a surface of the topmost IMD layer 24. This light is transmitted through the IMD layer 22 and the ILD layer 20 down to the photodiodes 11, 12 and 13 and a consequent electric signal is induced. The incident light 32 and 34 often strikes the surface of the IMD layer 24 at a variety of angles. For instance, the incident light 32 strikes the surface at a near perpendicular angle, and the incident light 34 strikes the surface at a non-perpendicular angle. The incident light 32 that strikes the surface of the IMD layer 24 at a perpendicular angle is transmitted to the photodiode 12 underlying the strike location. This is optimal for image sensing performance. However, the incident light 34 that strikes the surface of the IMD layer 24 at a non-perpendicular angle is transmitted to the metal line 25. The incident light 34 then reflects off the surface of the metal line 25 and is transmitted to the nearby photodiode 13 rather than to the photodiode 12 due to light scattering. This effect is called “crosstalk.” The light scattering problem causes interference in the photodiode 13 from the incident light 34, and results in a degraded signal-to-noise ratio (SNR). And, obviously, the crosstalk effect reduces CIS imaging sensitivity.
To overcome the crosstalk problem, U.S. Pat. No. 6,861,866 provides an improved CIS structure. FIG. 2 is a schematic diagram of a CIS structure according to U.S. Pat. No. 6,861,866. The CIS structure is defined as an image sensing area 40 on the left and an interconnect area 42 on the right. The CIS structure includes a substrate 44, a plurality of IMD layers 46, and a plurality of diffusion barrier layers 48 positioned on the substrate 44. The interconnects area 42 has a plurality of metal lines 50, which connect to a gate 52 and a source/drain 54 to control signal transduction from the CIS. The image sensing area 40 includes a photodiode 56 disposed on a surface of the substrate 44 and a light passageway 58 positioned above the photodiode 56. The light passageway 58 has an inside wall 62 formed by a plurality of metal barriers 60, a protective layer 64 disposed on the inside wall 62 and a surface of the IMD layers 46 to prevent the crosstalk effect, and a transparent filler 66 embedded in the light passageway 58. The image sensing area 40 also has a color filter 68 disposed on the transparent filler 66 and a microlens 70 formed above the light passageway 58. However, the inside wall 62 is formed by connecting the metal barriers 60 of each layer, and thus has a discontinuous surface that tends to cause light scattering. Consequently, the photodiode 56 receives little valid light.
U.S. Pat. No. 6,969,899 also discloses a similar CIS structure. Please refer to FIG. 3, wherein a CIS structure has a substrate 72, a plurality of photodiodes 74 formed therein, and a plurality of STI 76 separating the photodiodes 74. The CIS structure also has a plurality of first dielectric layers 78 covering the substrate 72, and a plurality of light passageways 80 connected to the photodiodes 74. Each light passageway 80 includes a second dielectric layer 82 filled therein and a third dielectric layer 84 disposed on an inside wall of the light passageway 80 to prevent the crosstalk effect. Since the light passageways 80 connect directly to the photodiodes 74, the sensing surface of the photodiode 74 is easily damaged by plasma damage and impurities during fabrication. This results in surface defects and increases leakage current. Sensitivity of photodiodes 74 is therefore reduced, and the photodiodes 74 may even lose functionality.